The previous hierarchical system of codes has been replaced as from January 2011[1] by a nine-character code for all types of geography, in which there is no relation between the code for a lower-tier area and the corresponding parent area. The older coding system has now been phased out.[2]

Electoral wards (called electoral divisions in Wales). These are the areas defined for the election of local councillors, but are also widely used for presenting statistics at a smaller scale than the whole district.

Census output areas (OA). These are the smallest unit for which census data are published - they were initially generated to support publication of 2001 Census outputs and contain at least 40 households and 100 persons, the target size being 125 households. They were built up from postcode blocks after the census data were available, with the intention of standardising population sizes, geographical shape and social homogeneity (in terms of dwelling types and housing tenure). The OAs generated in 2001 were retained as far as possible for the publication of outputs from the 2011 Census (less than 3% were changed[3]). Before 2001, census data were published for larger Enumeration Districts (ED) which were delineated before the census was conducted and were the organisational units for census data collection.

Super Output Areas (SOAs) are a set of geographical areas developed following the 2001 census, initially to facilitate the calculation of the Indices of Deprivation 2004 and subsequently for a range of additional Neighbourhood Statistics (NeSS). The aim was to produce a set of areas of consistent size, whose boundaries would not change (unlike electoral wards), suitable for the publication of data such as the Indices of Deprivation. They are an aggregation of adjacent Output Areas with similar social characteristics. Lower Layer Super Output Areas (LSOAs) typically contain 4 to 6 OAs with a population of around 1500. Middle Layer Super Output Areas (MSOAs) on average have a population of 7,200.[4] The hierarchy of Output Areas and the two tiers of Super Output Areas have become known as the Neighbourhood Statistics Geography.

Local Government wards were given a two-letter code within their local authority.

For example, 12UBGA for Petersfield Ward within Cambridge district.

The smallest level, Census OAs were originally given an additional 4 digits within a ward, so that the first output area in Petersfield Ward was coded 12UBGA0001.

Civil parishes were also coded using this hierarchical system. Parishes were coded using an additional 3 digits after their local authority. For example, within 12UD for Fenland district, the parish of Tydd St. Giles was coded 12UD010.

The current system replaces these codes with a fixed length code of nine characters. The first three characters indicate the level of geography and the six digits following define the individual unit. For example, the Royal Borough of Greenwich is coded as E09000011, Middlesbrough is E06000002, Cambridge E07000008 and Fenland E07000010.

The meaning of some common three character prefixes is as follows:[5][6]